This invention relates to a spinel single crystal substrate of the magnesium aluminate class for epitaxial growth thereon of a semiconductor layer. The substrate according to this invention therefore is for use in manufacturing semiconductor devices by the use of the epitaxially grown semiconductor layer.
A substrate of an electrically insulating material for epitaxial growth thereon of a semiconductor layer is theoretically welcomed because the substrate affords excellent electric insulation between the semiconductor device elements manufactured by the use of the semiconductor layer and eliminates the undesirable parastic capacity to thereby reduce the power consumption, enable the semiconductor devices to be manufactured in the form of an integrated circuit, and improve the high-frequency performance. A conventional insulator substrate consisting either of a sapphire or a spinel single crystal, however, shows appreciable mismatch as will be described hereunder with reference to FIG. 1 of the accompanying drawing between the crystal lattices of the substrate and the semiconductor single crystal epitaxially grown thereon in the form of the layer as called herein. It follows therefore that the mismatch seriously damages the electric characteristics of the semiconductor devices to reduce the yield of the products. In addition, the component aluminium of the sapphire or spinel tends to diffuse into the epitaxially growing semiconductor layer to render the thereby grown semiconductor single crystal heterogeneous.
An is known in the crystallography, spinel crystals consist essentially of magnesium aluminate which may be represented in chemical formula by MgO.nAl.sub.2 O.sub.3 wherein the mol ratio n of Al.sub.2 O.sub.3 to MgO is from unity to about four. A spinel crystal of this essential composition is termed herein a magnesium aluminate class spinel crystal.
In the meantime, Jun-ichi Nishizawa, one of the instant inventors, has proposed in Japanese Patent Application No. 47-74483 filed on July 25, 1972, and laid open to public on Mar. 28, 1974, to reduce the mismatch in question by changing the composition of at least one of the substrate and the semiconductor. In conjunction with a combination of a spinel substrate and a silicon layer formed thereon, it is described in the referenced Japanese patent application, among others, that the spinel should preferably have a larger lattice constant, that a greater lattice constant is expected to result from introduction into a spinel of at least one metal element selected from a group consisting of magnesium, nickel, tin, germanium, silicon, titanium, zirconium, hafnium, and beryllium, that a magnesium aluminate class spinel including MgO in excess, having the above-mentioned at least one element at the magnesium site, such as Sn.sub.1-x Mg.sub.x O.Al.sub.2 O.sub.3, or having the above-mentioned at least one element at the aluminium site, such as MgO.(Al.sub.1-x Si.sub.x).sub.2 O.sub.3, is expected to serve well, that addition to a magnesium aluminate class spinel of another spinel, such as TiZn.sub.2 O.sub.4 whose lattice constant is said to be a = 8.445 A, is thought to be effective, and that addition to a spinel of a chemical element of Group IV, such as silicon, germanium, or tin, is believed to be harmless to the silicon layer even autodoped. The specification of the referenced application thus points out verious possibilities but does not definitely teach as for the magnesium aluminate class spinel what substance should be added thereto in what quantity.